The present invention relates to communications and, more particularly, relates to spreading data, transmitted over a communications medium over a frequency range and over time in order to reduce negative effects of channel fading and noise on the communications medium and to improve the performance of error correction decoders.
It is a goal of communications to maximize rates of data transmission over a communications medium for a given signal to noise ratio and error rate. Typically, the bandwidth of the communications medium is limitedxe2x80x94either inherently, based on its electromagnetic properties, or for other reasons, such as governmental regulations. Thus communications systems must make efficient use of available bandwidth. Moreover, most communications mediums are susceptible to noise. Therefore, communications systems generally must include precautions to ensure reliable data transmission despite the noise on the communications medium.
One type of noise is selective fading. Selective fading is noise or interference that affects only a portion of a frequency range. Selective fading may completely take down a communications system that relies on only a single carrier frequency for data transmission when the selective fading occurs at the single carrier frequency.
In order to combat selective fading, many communications systems incorporate multiple carrier modulation (MCM), where multiple carriers are modulated and transmitted instead of a single carrier. In multiple carrier modulation (MCM) schemes, selective fading may impair some carriers, while other carriers at different frequencies remain unimpaired. Therefore, assuming data redundancy and error correction across the multiple carriers, which is typically the case, source data from the transmission system may still be recovered by the receiving system from the unimpaired subcarriers.
Another type of fading is commonly referred to as flat fading. Flat fading is interference or noise which affects all frequencies within a bandwidth of interest nearly equally. Flat fading, when severe enough, may completely impair a MCM communications system. This is because all of the carriers may be impaired, thus preventing any data from being properly received for the duration of the flat fading.
Typically, flat fading occurs during a finite interval of time. Data transmitted during the interval of flat fading is typically lost, or severely degraded in quality. Depending on requirements, such data may need to be retransmitted, resulting in potentially substantial and unacceptable delays. Alternatively, the quality of data at the receiving end of the communications system may suffer. It the case of radio, television, or voice transmission, the degraded quality would manifest itself as audible or visible static.
It would be desirable to spread data for transmission in a communications system over time as well as frequency, so that redundant data spans over a typical flat fading interval. It would further be desirable to recover transmitted data that has been spread in time at the receiving end of a communications system. It would further be desirable to recover data at a receiver of a MCM communications system that has undergone a flat fading interval, without requiring retransmission of the data or without an unacceptable error rate.
According to the present invention, a transmission system encodes blocks of source data with error correction codes and spreads encoded data for each source block over a range of time and frequencies. This has the effect of reducing the impact of flat fading because if a portion of the encoded data corresponding to a block of source data is lost as a result of flat fading during a time interval, other portions, transmitted at a different time, may not be affected. A receiving system may therefore recover the original block of source data based on the portions of the encoded data that were received, including error correction bits.
A transmission system according to the present invention includes one or more encoders, a plurality of interleavers, at least one delay unit and a signal generation unit. The encoder receives blocks of source data from one or more data sources. The encoder derives error correction codes for each block of source data and outputs a plurality of streams of data which include the error correction codes.
The interleavers reorder each data stream into an interleaved stream. The delay units delay at least one of the data streams relative to the other data streams. When the transmission system is an orthogonal frequency division multiplexed (OFDM) transmission system, each delay unit typically inserts a delay that is an integer multiple of an OFDM frame.
The signal generation unit modulates a plurality of sub-carriers, which may be OFDM sub-carriers, based on the interleaved substreams and upconverts the modulated sub-carriers for transmission.
A receiving system according to the present invention includes a receiver, at least one delay unit, a plurality of de-interleavers and a decoder. The receiver down-converts, de-modulates and de-multiplexes a received signal into a plurality of interleaved substreams. The delay units delay at least one of the substreams relative to the other substreams. The delay for each substream is chosen to equalize a total delay added to each substream, the total delay being a sum of delay added by the transmission and receiving systems.
The interleavers reorder each stream into a plurality of serial data streams. The decoder receives the serial data streams, corrects errors present in the serial data streams and outputs decoded data.